1. Field of Invention
This invention relates to the production of protective coatings for carbonaceous components of electrolytic cells used in the production of aluminum. The invention more specifically relates to coating compositions which provide carbonaceous components of electrolytic cells with protection from deterioration during electrolysis and components containing the same.
2. Description of Related Art
The manufacture of aluminum is conducted conventionally by the Hall-Heroult electrolytic reduction process, whereby alumina is dissolved in molten cryolite and electrolyzed at temperatures of about 900 to 1000xc2x0 C. This process is conducted in a reduction cell typically comprising a steel shell provided with an insulating lining of suitable refractory material, which is in turn provided with a lining of carbon which contacts the molten constituents. One or more anodes, typically made of prebaked carbon blocks, are connected to the positive pole of a direct current source, and suspended within the cell. One or more conductor bars connected to the negative pole of the direct current source are embedded in the carbon cathode substrate comprising the floor of the cell, thus causing the cathode substrate to become cathodic upon application of current.
Prebaked anodes used in the production of aluminum are comprised of an aggregate of petroleum coke with pitch as a binder, while the carbon lining is typically constructed from an array of prebaked cathode blocks, rammed together with a mixture typically comprising of anthracite, tar, and coal tar pitch.
Aluminum is produced in a molten form within an electrolysis cell as a result of the following reaction:
2Al2O3+3Cxe2x86x924Al+3CO2
In the conventional design of the Hall-Heroult cell, aluminum collects as a pool of molten aluminum along the base of the cell. In doing so, oxygen becomes liberated and reacts with the available carbon on the surface of the anodes to produce carbon dioxide gas. Theoretically, 0.334 kg of anodic carbon is consumed per kilo of aluminum produced as represented by the above reaction. In reality, however, anodic consumption is 25-35% greater.
Excess consumption of the prebaked anodes is the result of a series of secondary reactions, which can be summarized as follows:
i) Air oxidation: oxidizing reactions result from oxygen in the air contacting the upper part of the anode and, if the anode is left unprotected, reacting to produce carbon dioxide;
ii) Boudouard reaction: carbo-oxidation reactions result from CO2 at the surface of the anode being immersed in the electrolyte and producing carbon monoxide (known as the Boudouard equilibrium); and
iii) Dusting: the selective oxidation of pitch coke with respect to petroleum coke, results in the release of carbon particles, generating dust, which has negative effects on the operation.
The loss effected by such secondary reactions within the electrolytic cell amounts to approximately 10% of the production cost of aluminum.
The economic inefficiencies of aluminum production can be further attributed to the deterioration of the carbon lining or cathodic material of the electrolytic cell as a result of erosion and penetration of electrolyte and liquid aluminum, as well as intercalation by metallic sodium.
Although the Hall-Heroult process for aluminum production is the most reliable to date, there is a continual need for improvement. In view of the economic impact of the inefficiencies of this process, considerable effort has focused on the development of improved electrolytic cell components which are capable of withstanding the harsh conditions imposed by the electrolysis of aluminum.
For instance, U.S. Pat. No. 3,852,107 to Lorkin et al. teaches of an impermeable protective coating for electrodes comprising a matrix having a melting point under 1000xc2x0 C. and a refractory filler, dissolved or suspended in a liquid carrier such as water. As an example, the matrix component of this coating was described as a graphite wettable material such as boric acid and/or a glaze forming material such as sodium aluminum fluoride. Suggested refractory fillers include oxides, carbides, nitrides or borides. The use of a suitable surface tension modifying agent such as chrome ore was suggested in certain situations to improve the wetting of the graphite.
U.S. Pat. No. 4,624,766 to Boxall, et al. describes an aluminum wettable, cured, carbonized cathode material for use in aluminum electrolysis cells, comprising a hard refractory material in a carbonaceous matrix which includes a carbonaceous filler and carbon fiber bonded by a non-graphitized amorphous carbon, this matrix having a rate of ablation essentially equal to the rate of wear and dissolution of the refractory hard material in the operating environment of the cell.
Sekhar et al., WO 98/17842 published Apr. 30, 1998, describes a method for applying a refractory boride to components of an aluminum electrolysis cell by forming a slurry of particulate preformed refractory boride in at least two grades of colloidal carriers selected from the group consisting of colloidal alumina, yttria, ceria, thoria, zirconia, magnesia, lithia, monoaluminum phosphate, cerium acetate and mixtures thereof, the two colloidal carriers preferably each being of the same colloid, followed by drying. The two grades of colloidal carrier have mean particle sizes which differ from one another by about 10-50 nanometers.
U.S. Pat. No. 5,486,278 to Manganiello discloses a method of impregnating a carbonaceous cell component with a boron-containing solution to improve resistance to deterioration during cell operation. When water was used as the solvent for the boron-containing solution, a surfactant was required to achieve an acceptable treatment time. Alternatively, the solvent could be chosen from methanol, ethylene glycol, glycerin and mixtures thereof. This method required the intake of the boron-containing solution to a depth of 1-10 cm into the component to be protected. This patent further disclosed that the air oxidation of carbonaceous components treated in this manner was comparable to the net consumption of similar components treated with traditional aluminum protective coatings.
Despite previous efforts, conventional techniques for performing the electrolysis of aluminum are still employed most often. This indicates that a more technically superior or economically profitable method of combating carbonaceous cell component deterioration is not known.
Lignosulfonates, such as ammonium lignosulfonate, have long been used as binders in a variety of different industries but not in aluminum electrolysis cells.
It is an object of the present invention to provide an effective and economical method of treating components of an electrolytic cell, for producing aluminum, to protect them from deterioration during operation of the cell.
The present invention in its broadest aspect relates to a method of treating a carbonaceous cell component of an electrolytic cell for the production of aluminum, to improve the resistance of the component to deterioration during operation of the cell. The method comprises preparing a liquid suspension of refractory material dispersed in a lignosulfonate binder solution and applying the liquid as a protective coating to the carbonaceous cell component, followed by drying the coating. The refractory material may be selected from a wide variety of refractory compounds, such as boron, zirconium, vanadium, hafnium, niobium, tantalum, chromium and molybdenum compounds.
As a by-product of the pulp and paper industry, lignosulfonate is both abundant and relatively inexpensive. It has been found to be surprisingly effective as a binder in the harsh environment of an aluminum electrolysis cell.
According to one embodiment of the invention, lignosulfonate binder is used in the coating of prebaked carbon anodes. For this purpose, a liquid suspension is prepared of a boron compound, e.g. boric acid, boron oxide, hydrated boron oxide or borax, aluminum fluoride and a lignosulfonate binder, e.g. ammonium or calcium lignosulfonate, and the liquid suspension is applied as a protective coating on the anode. Typically, it is applied to the portions of the anode which are exposed to the atmosphere during cell operation. Following application, the coating is dried, e.g. by air drying at room temperature. For greater coating strength, the suspension may also include a phenolic resin binder.
In accordance with a further embodiment of the invention, the lignosulfonate binder is used for coating carbon cathode structures of an aluminum electrolysis cell. For this purpose, a liquid suspension is prepared of a refractory boride, e.g. titanium diboride, a lignosulfonate binder and a phenolic resin binder. This liquid suspension is then applied as a protective coating to the cathode structure, followed by drying.
As the formulation base of the liquid suspensions of the present invention, lignosulfonate acts as a dispersant for dispersing the ingredients in the bulk liquid state, a wetting agent for even application of the coating and a binder to create a continuous layer of suspended solids which effectively adheres to the carbonaceous surface.
Oxidation of the upper part of prebaked anodes during cell operation is one of the principal reasons for excess net carbon consumption. In general, prebaked anodes are covered with alumina, crushed bath or a mix thereof to protect them against air oxidation. The practice of applying an aluminum coating to anodic components in Hall-Heroult cell, to reduce the rate of air oxidation is widely used in aluminum production. However, this practice is not optimal with a net carbon consumption of approximately 410-460 kg/t Al. Not to mention, the exorbitant costs associated with aluminum coatings.
One preferred embodiment of the present invention provides a mixture of a boron compound, e.g. boric acid, boron oxide, hydrated boron oxide or borax, and aluminum fluoride dispersed in a lignosulfonate binder as a viscous liquid. In this form, the liquid may be applied to the surface of an anodic surface by pulverization (spraying). Upon drying, a protective coating exists which is capable of combating deterioration of the anode by oxidation. This viscous liquid can be applied to the upper one-half to one-third region of a prebaked anode at ambient temperature with an air gun at 120 psi pressure and allowed to dry at room temperature for approximately 3 hours. The coating is preferably applied over a general thickness range of 0.5 to 2mm. Application of the coating to an approximate thickness of 1 mm is most preferred.
The viscous coating liquid typically contains about 20 to 60% by weight of a 50% lignosulfonate solution, 25 to 60% by weight of boric acid and 0 to 25% by weight of aluminum fluoride. A preferred range is 20 to 40% lignosulfonate (50% solution), 30 to 55% boric acid and 0 to 15% aluminum fluoride. A particularly preferred range is 25-35% lignosulfonate (50% solution), 35-55% boric acid and 0-10% aluminum fluoride. The coating liquid may also contain up to 20% by weight of phenolic resin.
During operation in an aluminum electrolysis cell, the temperature of the top of the anodes in the cell reach approximately 550 to 650xc2x0 C. When coated with the above viscous coating liquid, and dried, the anodes are protected against oxidation by the formation of a boron and aluminum oxide coating on the anode.
A significant decrease in net carbon consumption is estimated for anodic components, having the protective coating as taught by this invention. It is estimated that the coating composition of this invention provides a savings of approximately $3 per ton of metal produced for each percent decrease in net carbon consumption.
Another preferred embodiment of the invention relates to a process for protecting the exposed surface of cathode blocks in an aluminum electrolysis cell, by applying a coating comprising titanium diboride dispersed in a mixture of lignosulfonate and phenolic resin. Such a coating provides wetting properties and erosion resistance as well as significantly reducing the deterioration of the underlying layers due to sodium and bath penetration. This coating mixture typically contains about 5 to 40% by weight lignosulfonate (50% solution), about 5 to 40% by weight phenolic resin, about 20 to 70% by weight titanium diboride and 0 to 5% anthracite (or graphite). A preferred composition contains about 14 to 20% lignosulfonate (50%), about 14 to 20% by weight phenolic resin and about 50 to 70% by weight titanium diboride and 2% to 5% by weight anthracite ( less than 74 micron). While titanium boride is the preferred material for this purpose, a wide variety of borides may be used, e.g. zirconium, vanadium, hafnium, niobium, tantalum, chromium or molybdenum boride.
This coating mixture is preferably applied to a thickness of about 1-3 mm with a spray gun at 120 psi pressure and the coated cathode is first air dried at room temperature for about 10 hours. Although, it is possible to increase the lifetime of the coating by increasing the thickness to 10-15 mm by applying many layers of the coating. Between each layer, the coating could be dried by a heating system at about 100-150xc2x0 C. The coated cathode is then preheated as a part of normal cell start-up. In preparation for the preheat, the cathode is covered with a 4 inch layer of coke (no bath) and the anodes are lowered until they rest on the coke layer. A current is then applied and under these conditions the coating will reach a temperature of about 1000xc2x0 C., in about 25 hours.
The above composition provides a wettable surface for the metal and not only protects the exposed cathode surface from deterioration, but also reduces the absorption of sodium by the cathode lining in general and reduces the oxidation of the side wall blocks, when applied to these areas.